The present invention relates to a color cathode ray tube, and in particular, a color cathode ray tube having reinforcing beads which increase a curved surface-holding strength of a shadow mask.
In general, a color cathode ray tube is provided with an envelope which comprises a funnel and a substantially rectangular panel including an effective portion having a curved surface and a skirt portion provided at a peripheral portion of the effective portion. A phosphor screen is formed on the inner surface of the effective portion of the panel, and comprises three-color phosphor layers which emit three light components of blue, green, and red. A substantially rectangular shadow mask is located inward of and opposite to the phosphor screen. Furthermore, an electron gun for emitting three electron beams is arranged within a neck of the funnel.
In the color cathode ray tube, three electron beams emitted from the electron gun are deflected by means of a deflecting unit mounted on the outer side of the funnel, and horizontally and vertically scans over the phosphor screen through the shadow mask, thereby displaying a color image.
The shadow mask is intended to select the three electron beams emitted from the electron gun so that they are correctly incident on the three-color phosphor layers. The shadow mask comprises a substantially rectangular mask body and a substantially rectangular mask frame attached to a skirt portion of the mask body. The mask body has an effective surface and the skirt portion provided at a peripheral area of the effective surface. In the effective surface, a large number of electron beam passage apertures are arranged in a predetermined manner, and allow electron beams to pass therethrough. The effective surface is constituted by a curved surface opposite to the phosphor screen.
In the color cathode ray tube having the above structure, the electron beams passing through the electron beam passage apertures of the shadow mask must be correctly landed on the three-color phosphor layers, in order to display an image with no color deviation on the phosphor screen. Thus, it is necessary to maintain the correct positional relationship between the panel and the shadow mask.
In recent years, in color cathode ray tubes, the effective portion of the panel has been flattened, and as a result, the effective surface of the shadow mask has also been flattened, reducing the curvature of the effective surface. In such a manner, when the curvature decreases, the strength of the shadow mask lowers, as a result of which there is a possibility of the shadow mask being deformed in the manufacturing process of the color cathode ray tube. When the shadow mask is deformed, the positional relationship between the shadow mask and the panel varies, and the color purity of a displayed image greatly deteriorated.
Furthermore, when the color cathode ray tube operates, parts of the electron beams emitted from the electron gun impinge on the shadow mask, and are then converted into heat energy, heating the shadow mask. As a result, a doming of the shadow mask is generated. To be more specific, it thermally expands toward the phosphor screen. Due to doming, when the distance between the effective surface of the shadow mask and the inner surface of the panel varies, and exceeds an allowed range, electron beams mis-land on the phosphor layers, thus lowering the color purity. In particular, when an image pattern having a high luminance is locally displayed, doming of the shadow mask locally occurs, and local mis-landing is brought about in a short time period. In addition, as a result of flattening the shadow mask as mentioned above, doming further easily occurs as the curvature of the effective surface of the shadow mask decreases.
U.S. Pat. No. 5,506,466 discloses a shadow mask wherein steps (reinforcing beads) are provided at an effective surface, as means for preventing lowering of image quality which is caused by deformation of the shadow mask and local doming thereof. In other words, the reinforcing beads are intended to improve the curved surface-holding strength of the shadow mask, and as mentioned above, prevent local doming and deformation of the shadow mask in the manufacturing process.
However, if the reinforcing beads are applied to a shadow mask provided with an effective surface having a small curvature, and its position and height are set in such a way to achieve a sufficient effect, the distance between the effective surface of the shadow mask and the inner surface of the effective portion of the panel changes locally. Therefore, when a phosphor screen is formed by a photographic printing method using the above shadow mask as a photomask, the reinforcing beads influence to the photographic printing of the phosphor screen, and as a result, the appearance quality of the phosphor screen remarkably lowers. Considering lowering of the appearance quality, the height of the step or the reinforcing bead is limited to about 0.1 to 0.20 m. However, if the height is limited to such a degree, it is difficult to sufficiently increase the curved surface-holding strength of the shadow mask having an effective surface having a small curvature.